12   4 The phenotype of an organism is affected by its genotype. (a) The shape of the eye in fruit flies is controlled by a single gene. Fruit flies that are homozygous for the allele R have round eyes. Fruit flies that are homozygous for the allele B have bar eyes. The diagram shows the eyes on the heads of the fruit flies. round-eyed fruit fly bar-eyed fruit fly wide bar-eyed fruit fly eye (i) State what is meant by the term allele. (2) ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... Turn over 13   (ii) When a round‑eyed fruit fly is crossed with a bar‑eyed fruit fly, offspring are produced that have wide bar eyes. Explain why offspring are produced with the wide bar eye shape. (3) ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... (b) Tay‑Sachs disease is a genetic condition. Individuals who are homozygous recessive for this condition usually survive for up to four years. (i) Determine the probability of two individuals who are heterozygous for the Tay‑Sachs gene having a child with this condition. Use a genetic diagram, and the alleles T and t. (2) Probability .............................................................. 14   (ii) An embryo, created by IVF, can be screened for Tay‑Sachs disease before being placed in the mother’s uterus. Name this type of genetic screening. (1) ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... (iii) Explain why the mother may prefer this type of prenatal screening to amniocentesis. (2) ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... ..................................................................................................................................................................................................................................................................................... (Total for Question 4 = 10 marks) Turn over 15   BLANK PAGE